Balancing Chemical Equations

1. Balancing Chemical Equations What goes in must come out!

2. Balancing Chemical Equations • Balancing a chemical equation is much like the work of an accountant who has to show every penny that comes in and where it has gone to.

3. Objectives • Learn the steps to balancing chemical equations. • Take notes to help you understand. • Test yourself with a set of equations to balance. • Enter your own equations to see if they balance.

4. Law of Conservation of MassYou need to remember this law! • The Law of Conservation of Mass states: that mass is neither created nor destroyed in any chemical reaction. Therefore balancing of equations requires the same number of atoms on both sides of a chemical reaction. • The number of atoms in the Reactants must equal the Number of atoms in the Products

5. Lavoisier, 1788 Chemical Equations Because of the principle of the Conservation of Matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides.

6. Law of Conservation of Mass • The mass of all the reactants (the substances going into a reaction) must equal the mass of the products (the substances produced by the reaction). • Reactant + Reactant = Product

7. A simple equation, such as the synthesis of Iron (II) sulfide, • iron + sulfur Iron (II) sulfide • Fe + S FeS • Note that in a chemical equation, by convention, we use the arrow “ " instead of the equals “ = ".

8. Balancing Equations ___ Al + ___ Br2 ---> ___ Al2Br6 2 3

9. Write all reactants on the left and all products on the right side of the equation arrow. Make sure you write the correct formula for each element 2. Use coefficients in front of each formula to balance the number of atoms on each side. Steps to Balancing a Chemical Equation

10. 3. Multiply the coefficient of each element by the subscript of the element to count the atoms. Then list the number of atoms of each element on each side. 4. It is often easiest to start balancing with an element that appears only once on each side of the arrow. These elements must have the same coefficient. Next balance elements that appear only once on each side but have different numbers of atoms. Finally balance elements that are in two formulas in the same side. Steps to Balancing a Chemical Equation

11. Re-cap of steps from rule 4: • Balance elements that appear only once on each side of the arrow. • Next balance elements that appear only once on each side but have different numbers of atoms. • Finally balance elements that are in two formulas in the same side.

12. Balancing Chemical Equations An easier way

13. Then all you do is list the atoms that are involved on each side of the arrow Mg + O2 MgO First you need an equation with the correct “formulae” ………. You’ll probably be given this in the question Just like this one Mg + O2 MgO Mg O Mg O

14. Mg + O2 MgO Mg + O2 MgO Mg O Mg O 1 1 2 1 Then start balancing: [1] Just count up the atoms on each side 1 1 2 1 [2] The numbers aren’t balanced so then add “BIG” numbers to make up for any shortages 2 2 2 And adjust totals

15. 2 2 1 Mg + O2 MgO 2 2 Mg O But the numbers still aren’t equal, so add another “BIG” number 2 2 And adjust totals again NOW BOTH SIDES HAVE EQUAL NUMBERS OF ATOMS WE SAY THAT THE EQUATION IS BALANCED!!

16. Try to balance these equations using the same method: [1] Na + Cl2 NaCl [2] CH4 + O2 CO2 + H2O [3] Li + HNO3  LiNO3 + H2 [4] Al + O2 Al2O3

17. How did you get on?? Here are the answers: [1] 2 Na + Cl22 NaCl [2] CH4 + 2 O2 CO2 + 2 H2O [3] 2 Li + 2 HNO3 2 LiNO3 + H2 [4] 4 Al + 3 O22 Al2O3

18. Example • NH3 + O2 NO + H2O ReactantsProducts • N appears once on both sides in equal numbers, so the coefficient for NH3 is the same as for NO.

19. Example: NH3 + O2 NO + H2O • Next look at H which appears only once on each side but has different numbers of atoms, 3 on the left and 2 on the right. The least common multiple of 3 and 2 is 6, so rewrite the equation to get 6 atoms of H on both sides: • 2NH3 + O2 NO + 3H2O

20. Example: 2NH3 + O2 NO + 3H2O • There are 2 oxygen atoms on the left and 5 on the right — the least common multiple of 2 and 5 is 10, so rewrite the equation as: • 2NH3 + 5O24NO + 6H2O

21. Now count the atoms on each side: • 2NH3 + 5O2 4NO + 6H2O • Write them out keeping them on the appropriate side of the chemical equation • 2 N (nitrogen atoms)4 N (nitrogen atoms) • 6 H (hydrogen atoms) 12 H (hydrogen atoms) • 10 O (oxygen atoms)10 O (oxygen atoms) • This shows the equation not to be balanced “YET”

22. Check the number again: • If you double the N and H on the left the equation will be balanced: • 4NH3 + 5O2 4NO + 6H2O

23. Double-check: • 4NH3 + 5O2 4NO + 6H2O • 4 N (nitrogen atoms)4 N (nitrogen atoms) • 12 H (hydrogen atoms)12 H (hydrogen atoms) • 10 O (oxygen atoms)10 O (oxygen atoms) • The equation is Balanced

24. Balancing Practice • For more help go to: http://richardbowles.tripod.com/chemistry/balance.htm#part0 • For some fun balancing equations go to: • http://www.mpcfaculty.net/mark_bishop/balancing_equations_tutorial.htm